Remove the watch on the /vm/<uuid> path when rebooting or migrating. We don't

want to end up with multiple watches on the same path, otherwise we get a
watch event storm after many such reboots or migrates.

Signed-off-by: Ewan Mellor <ewan@xensource.com>

tools/python/xen/xend/XendDomainInfo.py

@@ -775,7 +775,10 @@ class XendDomainInfo:
                     if reason == 'suspend':
                         self.state_set(STATE_DOM_SHUTDOWN)
                         # Don't destroy the domain.  XendCheckpoint will do
-                        # this once it has finished.
+                        # this once it has finished.  However, stop watching
+                        # the VM path now, otherwise we will end up with one
+                        # watch for the old domain, and one for the new.
+                        self.unwatchVm()
                     elif reason in ['poweroff', 'reboot']:
                         restart_reason = reason
                     else:
@@ -1185,19 +1188,32 @@ class XendDomainInfo:
     def cleanupVm(self):
         """Cleanup VM resources.  Idempotent.  Nothrow guarantee."""
 
-        try:
-            try:
-                if self.vmWatch:
-                    self.vmWatch.unwatch()
-                self.vmWatch = None
-            except:
-                log.exception("Unwatching VM path failed.")
+        self.unwatchVm()
 
+        try:
             self.removeVm()
         except:
             log.exception("Removing VM path failed.")
 
 
+    ## private:
+
+    def unwatchVm(self):
+        """Remove the watch on the VM path, if any.  Idempotent.  Nothrow
+        guarantee."""
+
+        try:
+            try:
+                if self.vmWatch:
+                    self.vmWatch.unwatch()
+            finally:
+                self.vmWatch = None
+        except:
+            log.exception("Unwatching VM path failed.")
+
+
+    ## public:
+
     def destroy(self):
         """Cleanup VM and destroy domain.  Nothrow guarantee."""
 
@@ -1351,6 +1367,7 @@ class XendDomainInfo:
             if rename:
                 self.preserveForRestart()
             else:
+                self.unwatchVm()
                 self.destroyDomain()
 
             # new_dom's VM will be the same as this domain's VM, except where
@@ -1387,6 +1404,7 @@ class XendDomainInfo:
         log.info("Renaming dead domain %s (%d, %s) to %s (%s).",
                  self.info['name'], self.domid, self.info['uuid'],
                  new_name, new_uuid)
+        self.unwatchVm()
         self.release_devices()
         self.info['name'] = new_name
         self.info['uuid'] = new_uuid
@@ -1398,6 +1416,7 @@ class XendDomainInfo:
     def preserve(self):
         log.info("Preserving dead domain %s (%d).", self.info['name'],
                  self.domid)
+        self.unwatchVm()
         self.storeDom('xend/shutdown_completed', 'True')
         self.state_set(STATE_DOM_SHUTDOWN)